Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus with a cover plate formed separately from a substrate table and means for stabilizing a temperature of the substrate table by controlling the temperature of the cover plate is disclosed. A lithographic apparatus with thermal insulation provided between a cover plate and a substrate table so that the cover plate acts as a thermal shield for the substrate table is disclosed. A lithographic apparatus comprising means to determine a substrate table distortion and improve position control of a substrate by reference to the substrate table distortion is disclosed.

1. FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

2. BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, United States patent U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another solution which has been proposed is to provide the liquid supplysystem with a barrier member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The barrier member is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the barrier member and thesurface of the substrate. In an embodiment, the seal is a contactlessseal such as a gas seal. Such a system with a gas seal is disclosed inU.S. patent application publication no. US 2004-0207824, herebyincorporated in its entirety by reference.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting the substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus may have only one table movable between exposure andmeasurement positions.

The substrate W may be displaced in the XY plane by moving the substratetable WT on which it is supported. The relative position of thesubstrate table WT and, therefore, the substrate W may be determined byreference to one or more mirrors mounted on lateral sides of thesubstrate table WT. For example, one or more interferometers may beprovided to measure a substantially perpendicular distance from pointson the surface of these mirrors to corresponding points or axes in areference frame. Distortion of the substrate table WT may cause thesemirrors to deform, thus compromising the accuracy with which thesubstrate W is moved and/or positioned relative to the projection systemPS, which may have a negative impact on the quality of a pattern to beformed on the substrate W.

3. SUMMARY

It is desirable, for example, to provide a system for improving theaccuracy with which a substrate may be positioned relative to, forexample, the projection system.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a substrate table arranged tosupport a substrate; a projection system configured to project amodulated radiation beam onto a substrate; a liquid supply systemconfigured to provide a liquid in a region between the projection systemand a substrate during exposure; a cover plate, physically separate fromthe substrate table, positioned radially outside of the substrate duringexposure, and configured to provide a surface facing the projectionsystem that is substantially adjacent to and level with the substrate;and a substrate table temperature stabilization device configured toreduce a temperature deviation of a part of the substrate table from acorresponding target temperature by controlling a temperature of a partof the cover plate.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising: projecting a modulatedradiation beam, through a liquid, onto a substrate held on a substratetable; and reducing a temperature deviation of a part of the substratetable from a corresponding target temperature by controlling atemperature of a part of a cover plate, the cover plate physicallyseparate from the substrate table, radially outside of the substrateduring projection of the modulated radiation beam and having a surfacethat is substantially adjacent and level with the substrate.

According to a further aspect of the invention, there is provided alithographic apparatus, comprising: a substrate table arranged tosupport a substrate; a projection system configured to project amodulated radiation beam onto a substrate; a liquid supply systemconfigured to provide a liquid in a region between the projection systemand a substrate during exposure; a cover plate, physically separate fromthe substrate table, positioned radially outside of the substrate duringexposure, and configured to provide a surface facing the projectionsystem that is substantially adjacent to and level with the substrate;and a thermal insulator arranged to reduce heat transfer between thecover plate and the substrate table in order to provide thermalshielding of the substrate table by the cover plate.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising: projecting a modulatedradiation beam, through a liquid, onto a substrate held on a substratetable; and thermally insulating the a cover plate so as to reduce heattransfer between the cover plate and the substrate table and therebyenable thermal shielding of the substrate table by the cover plate, thecover plate physically separate from the substrate table, radiallyoutside of the substrate during projection of the modulated radiationbeam and having a surface that is substantially adjacent and level withthe substrate.

According to a further aspect of the invention, there is provided Alithographic apparatus, comprising: a substrate table arranged tosupport a substrate; a projection system configured to project amodulated radiation beam onto a substrate; a measuring system configuredto determine a position of a portion of the substrate table; a substratetable distortion determining device arranged to provide data regarding adistortion of the substrate table; and a substrate position controllerconfigured to control the position of the substrate relative to theprojection system by reference to the position of a portion of thesubstrate table measured by the measuring system and data regarding adistortion of the substrate table provided by the substrate tabledistortion determining device.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising: projecting a modulatedradiation beam onto a substrate held by a substrate table; determining aposition of a portion of the substrate table; and controlling theposition of the substrate relative to a projection system used toproject the modulated radiation beam by reference to the determinedposition of the portion of the substrate table and data regarding adistortion of the substrate table.

According to a further aspect of the invention, there is provided amethod of mapping a surface profile of a substrate table reflector in alithographic apparatus, comprising: providing a first substantiallyplanar reflector mounted on a first lateral side of a substrate tableconfigured to support a substrate, the first reflector having a normalparallel to a first axis; providing a second substantially planarreflector mounted on a second lateral side of the substrate table, thesecond reflector having a normal parallel to a second axis non-parallelwith respect to the first axis; and moving the substrate table parallelto the first axis while measuring a perpendicular distance from asurface of the second reflector to a reference point in a referenceframe.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising: mapping a surface profile of areflector of a substrate table by moving the substrate table parallel toa first axis while measuring, in a direction substantially parallel to asecond axis, a distance from a surface of the reflector to a referencepoint, the second axis being substantially orthogonal to the first axis;projecting a modulated radiation beam onto a substrate; and moving thesubstrate relative to a projection system used to project the modulatedradiation beam in order to expose different target regions of thesubstrate, the movement controlled by reference to a position of thesubstrate, the position being determined by reference to a measurementof the separation of the substrate table reflector from a referencepoint and the surface profile of the substrate table reflector.

4. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a substrate mounted on a substrate holder and movablesubstrate table;

FIG. 6 depicts the effects on a substrate table caused by thermalexpansion of a substrate holder;

FIG. 7 depicts thermal insulation between a cover plate and thesubstrate table according to an embodiment of the invention;

FIG. 8 depicts a substrate table temperature stabilization deviceaccording to an embodiment of the invention, with a network of channels;

FIG. 9 depicts a substrate table temperature stabilization deviceaccording to an embodiment of the invention, with an electrical heaterand a controller;

FIG. 10 depicts a substrate table temperature stabilization deviceaccording to an embodiment of the invention, with heating elementsconfigured to be activated by a varying magnetic field;

FIG. 11 depicts a substrate table assembly incorporating a thermallyconducting coupling medium according to an embodiment of the invention;

FIG. 12 depicts a position measuring system and a substrate tabledistortion measuring device according to an embodiment of the invention;and

FIG. 13 depicts a measuring system comprising a plurality ofinterferometers per mirror according to an embodiment of the invention.

5. DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT an the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 5 schematically shows a substrate W supported by a substrate holder2 and substrate table WT. The substrate table WT can be moved relativeto the projection system PS so as to expose different target regions ofthe substrate W. Accurate positioning of the substrate W is generallydesirable. For example, where a device is to be formed from severallithographically produced layers, each layer should have a precisespatial correspondence with the other layers. The extent of thiscorrespondence depends on how well the substrate W is positioned duringpatterning of each of the layers.

Accurate displacement can be achieved, for example, using a feedback orservo loop in combination with means for physically moving the substratetable WT and means for measuring its position. The substrate W is movedtowards a target position by progressively reducing a difference betweena measured position and the target position in the shortest possibletime and, desirably, without overshoot.

Where the substrate W is fixed relative to, or has a known spatialrelationship with, the substrate table WT, it may be more convenient tomeasure the position of one or more points on the substrate table WT anddeduce therefrom the position of the substrate W than to try directly tomeasure the position of the much smaller and thinner substrate W.

The accuracy of such an approach depends at least partly on theprecision with which the position of the substrate W may be deduced frommeasurements of the substrate table WT. In particular, this may becomedifficult where the substrate table WT becomes distorted duringexposure, which may occur, for example, due to temperature variations ofthe substrate table WT and/or components that are in mechanical contactwith the substrate table WT.

Temperature variations may arise due to heating from the lithographyradiation, for example. Alternatively or additionally, particularly inimmersion systems, evaporation of liquid (normally immersion liquid)from the surface of the substrate W and surrounding regions may lead tocooling. The situation may be further complicated by compensationsystems that may be incorporated to control the temperature of thesubstrate W or other important elements. For example, a substrate heatermay be supplied to counteract the cooling caused by evaporation ofimmersion liquid. Although the heater may be designed to maintain thesubstrate W at a more constant temperature, it may lead to a greatertemperature gradient and/or variation in other components, such as thesubstrate table WT.

FIG. 5 shows an arrangement in which the substrate position may bemeasured indirectly via measurements of the substrate table WT. In theexample shown, the substrate W is supported via protrusions or “burls” 5on a substrate holder 2, itself configured to rest via burls 3 on thesubstrate table WT.

One or more cover plates 60 are provided to allow a liquid confinementstructure 68 (see FIG. 11, for example—the liquid confinement structure68 is not shown in FIGS. 5 and 8 to 10, for clarity), which at leastpartly confines an immersion liquid, to pass smoothly over the surfaceof the substrate W. In this configuration, the cover plate 60 ispositioned radially outside of the substrate W in order to provide asufficiently large planar surface for the liquid confinement structure68 to operate as the substrate W is scanned relative to the projectionsystem PS. The cover plate 60 may also be detachably mounted on burls 3to provide flexibility for substrates of different sizes.

One or more mirrors 6 are mounted on lateral sides of the substratetable WT and it is by means of these mirrors that the position of thesubstrate table WT is determined. For example, one or moreinterferometers may be used, operating on the basis of radiationreflected from these mirrors. The interferometer can use the reflectedradiation to deduce how far the surface of the mirror is from aparticular point in a reference frame that is fixed with respect to thedetecting part of the interferometer. In order to determine the positionof the substrate table WT along different axes, a plurality of mirrorsand corresponding interferometers may be provided. For example, twoplanar mirrors may be provided that face in orthogonal directions. Themirrors may be fixed to the substrate table WT or may be formedintegrally with the substrate table WT.

Distortion of the substrate table WT can cause a distortion in the shapeof the mirrors that are mounted on it. FIG. 6 schematically depicts atop view of the arrangement shown in FIG. 5 and illustrates one way inwhich such distortion may occur. In the left-hand figure, the centercircle represents the substrate W with the substrate holder 2 (notvisible) beneath it. The surrounding square is the substrate table WTwith substantially perfectly planar mirrors 6 mounted on its lateralsides. The right-hand figure shows (in exaggerated form) the effects ofthermally-induced distortion (arrows 10 in FIG. 5) of the substrateholder 2 on the substrate table WT. Heating of the substrate holder 2has caused it to expand from its original form (thin line) to athermally expanded form (thicker line; arrows 8 in FIG. 6). This heatingmay have been caused by a substrate heater 4 (which may be a pluralityof channels used to conduct a heat-exchange fluid, for example),configured perhaps to counteract the cooling effect on the substrate Wof immersion liquid evaporation. Alternatively or additionally, theheating may have arisen due to the lithography radiation itself.

The expanded substrate holder 2, which may be held quite firmly againstthe substrate table WT by, for example, a low pressure maintainedbetween the two components, may exert radial forces that cause adistortion (arrows 12 in FIG. 5) in the body of the substrate table WT.This distortion, in turn, may cause a corresponding deformation of themirrors 6 as shown in the right-hand diagram of FIG. 6 (thicker lines;arrows 14).

Either or both of the substrate table WT and substrate holder 2 may beformed from a material having a very small thermal expansioncoefficient. However, such materials may be expensive and may not beavailable in a wide range of physical properties, which may make themless suitable in other respects. For example, wear resistance may be animportant property for the substrate holder 2 but materials with goodwear resistance and near-zero thermal expansion coefficient may not beavailable and/or economical.

Although the substrate table WT distortion in FIGS. 5 and 6 is shown asarising from the expansion of the substrate holder 2, distortion mayalso arise due to heating or cooling of the substrate table WT itself.For example, evaporation of immersion liquid is likely to cause somedegree of thermal contraction. The substrate table WT may expand in someparts and contract in others leading to a distortion more complex thanthat shown in FIG. 6, which assumes a deformation based on a uniformexpansion of the substrate holder 2.

The distortion of the mirrors 6 may cause an error in the control of thesubstrate W. For example, if corrective measures are not taken, when aportion of the mirror that is bulging outwards is positioned adjacent tothe detecting interferometer, the interferometer may output a signalindicating that the substrate table WT as a whole is closer than itactually is, where in reality it is just a localized portion of themirror that is closer. This may cause an erroneous offset in theposition of the substrate W when this signal is input to the feedbackloop.

FIG. 7 schematically shows an embodiment wherein the cover plate 60 isarranged so as to provide a thermal shield against possible sources ofheating and/or cooling of the substrate table WT. This is achieved byproviding thermal insulation between the cover plate 60 and thesubstrate table WT. In this way, change in temperature of the coverplate 60, caused for example by evaporation of immersion liquid orradiative heating, do not lead to a significant transfer of heat to thesubstrate table WT below the cover plate 60. In the example shown, thethermal insulation takes the form of special burls 21. The thermalconductance of these burls 21 is arranged to be low by minimizing theircross-sectional size and/or by using material of low thermalconductivity, for example. Radiative heat exchange between the substratetable WT and cover plate 60 may be reduced by applying reflectivecoatings on either or both of these components.

FIG. 8 schematically shows part of a lithographic apparatus according toan embodiment of the invention, comprising a substrate table temperaturestabilization device which is configured to maintain the temperature ofa part of the substrate table WT and/or components in mechanical and/orthermal contact with the substrate table WT within a certain range (orranges) of a target temperature. The range will depend on the accuracydesired and on the sensitivity of the substrate table WT (and therelated components mentioned above) to change in temperature (e.g. itsthermal expansion coefficient, mechanical structure, and/or anyheat-sinking arrangements that are provided). Different ranges and/ortarget temperatures may be used for different parts of the substratetable WT according to an expected heat input or output. For example, atighter tolerance might be desirable in a region of the substrate tableWT that is more susceptible to thermal expansion or which is expected tobe less exposed to heat input/output. Additionally or alternatively, aplurality of different target temperatures for different parts of thesubstrate table WT may be used where it is appropriate to maintain acontrolled inhomogeneous temperature profile within the substrate tableWT.

Stabilizing the temperature of the substrate table WT and/or relatedcomponents may reduce the extent of thermal expansion and/or contractionthereof and thereby may reduce the overall distortion of the substratetable WT. This in turn may minimize disruption to the operation ofsubstrate table WT position measuring device, such as mirrors 6 (notshown in FIG. 8) and thereby may improve the accuracy with which thesubstrate W may be positioned relative to the projection system PS, thuspossibly reducing overlay errors for example.

The temperature stabilization device according to this embodimentoperates by controlling the temperature of one or more cover plates 60.This may be done actively by controlling the temperature profile(including spatially constant or spatially varying temperaturedistribution) of the cover plate 60 according to a measured variation inthe temperature profile or average temperature of the substrate tableWT. Alternatively, the temperature stabilization device may operate morepassively by merely controlling the temperature of the cover plate inisolation, without reference to measurement of the temperature profileof the substrate table WT (although the temperature of the cover platemay be measured). According to this latter approach, the temperature ofthe cover plate 60 may be controlled so as to maintain a substantiallyconstant temperature (or, put another way, to maintain the temperatureof the cover plate within a range of a target temperature for the coverplate 60). Controlling the temperature of the cover plate 60 withoutdirect reference to the temperature of the substrate table WT may beimplemented with a minimum of disruption to the substrate tableapparatus.

Temperature control of the cover plate 60, passively and/or actively(see above), may effectively “shield” the substrate table WT fromseveral of the most important sources of heat input and/or output. Forexample, a large proportion of evaporation of the immersion liquid,which causes cooling, may occur on the surface of the cover plate 60.Similarly, radiative heating from components exposed to the region abovethe substrate table WT will tend to impinge first on the cover plate 60.Controlling the temperature of the cover plate 60 directly to compensatefor one or more of these factors may mean that their eventual effect onthe temperature of the substrate table WT is reduced.

Where the cover plate temperature is controlled by reference to themeasured temperature of the substrate table WT, the cover platetemperature control can also at least partially compensate for heatingand/or cooling that reaches the substrate table WT without passing firstthrough the cover plate 60 (for example, heating from substratetemperature compensation apparatus installed in the substrate holder 2and/or from radiative heating from the radiation beam).

Temperature control of the cover plate 60 according to the arrangementof FIG. 8 is achieved by means of a network of channels 20 embeddedwithin or on the cover plate 60 and a controller 30 arranged to controlthe temperature and/or pressure (and, therefore, flow rate) of a heatexchange fluid flowing within the network of channels 20 in order tomaintain the temperature of a part of the substrate table WT (and/orrelated components such as substrate holder 2 and cover plate 60) withina range of a corresponding target temperature (each target temperature“corresponding” to one or more of the parts of the substrate table WT(and/or related components) that are being controlled). The heatexchange fluid may be purified water, for example. The temperatureand/or pressure may be controlled, for example, by reference tocalibration experiments, mathematical modelling of an expected powerinput/output to the substrate table WT, actual measurements of thetemperature of the components contributing to the distortion (seebelow), and/or actual measurements of the fluid.

The system of channels 20 can be configured both to heat the cover plate60 and to cool the cover plate 60, as required, thereby allowingflexible control of the substrate table temperature. Alternatively oradditionally, the channels 20 can be configured to heat a part of thecover plate 60 while cooling another part. This may be achieved when avariation in temperature across the cover plate 60 and/or substratetable WT (and/or other related components) spans the temperature of theheat-exchange fluid. Alternatively, a system may be provided to supplyheat-exchange fluid with one set of properties (e.g. high temperature)to one portion of the system of channels 20 while simultaneouslyproviding heat-exchange fluid with a different set of properties (e.g.lower temperature) to another portion of the system of channels 20. Inthis way, the system of channels can be used to stabilize a wide rangeof temperature variations in the cover plate 60 and/or substrate tableWT (and/or other related components).

Alternatively and/or additionally, the substrate table temperaturestabilization device may be provided with a one or more electricalheaters 26 and a controller 40 as schematically shown in FIG. 9. The oneor more electrical heaters 26 may be embedded within the cover plate 60as shown or may be attached to a surface of the cover plate 60 (abovethe cover plate, below it or on both sides).

An electrical heater can be controlled easily and with a minimum ofadditional hardware. Its output can be adjusted quickly, providingenhanced control and rapid response.

According to an embodiment, the heating elements 26 shown in FIG. 9 maycomprise a material that undergoes a temperature-induced phasetransition in the region of the target temperature, the phase transitioncausing the material to change from a state in which it produces arelatively high heating output below the transition temperature to astate in which it produces a relatively low heating output above thetransition temperature. For example, a material that undergoes amagnetic-ordering transition may be chosen, such as a ferro-magnet, ananti-ferromagnet or a ferri-magnet. Alternatively or additionally, amaterial that undergoes a structural phase transition may be chosen.

The material may be chosen so that the resistivity of the materialsuddenly increases as the material is heated through the transitiontemperature. If the controller 40 is configured to maintain a constantvoltage, the electrical power dissipated in the material will suddenlydecrease due to the sudden increase in resistivity, which will tend tostabilize the temperature of the substrate table WT, even when thetemperature varies strongly with position, without the need for complexcontrol circuitry and a large number of temperature sensors and heaters.Where the temperature is too low (i.e. below the transition temperatureand target temperature), the heating output will automatically berelatively high and where the temperature is too high (i.e. above thetransition temperature and target temperature), the heating output willautomatically be lower.

An alternative or additional approach, which operates according to ananalogous principle, is schematically shown in FIG. 10. Here, one ormore heating elements 26 are provided that are actuated by one or moreelectromagnets 28 controlled by controller 50. The one or more heatingelements 26 comprise a material that undergoes a phase transition thatcauses the material to change from a magnetically hysteretic state belowthe transition temperature to a non-magnetically-hysteretic state (i.e.a state which shows no, or a minimal amount of, magnetic hysteresis)above the transition temperature. The controller 50 and one or moreelectromagnets 28 are configured to apply a varying magnetic field tothe one or more heating elements 26, which will cause the one or moreheating elements to provide heat to the substrate table WT by magnetichysteresis, only if the one or more heating elements are below thetransition temperature. A ferromagnetic material may be used as themagnetically hysteretic material, for example. Again, this arrangementwill tend to stabilize even position-varying temperature variations ofthe cover plate 60 and/or substrate table WT (and/or other relatedcomponents) without the need for complex control circuitry and a largenumber of temperature sensors and heaters.

One or more temperature sensors 22 may be provided, fixed to thesubstrate table WT and/or cover plate 60, embedded within the substratetable WT and/or cover plate 60 (shown in FIGS. 8, 9 and 10), orpositioned adjacent to the substrate table WT and/or cover plate 60 (forexample, infra-red sensors). One or more temperature sensors may also beprovided in or on other components in thermal and/or mechanical contactwith the substrate table WT. The one or more temperature sensors provideinformation about the temperature of the substrate table WT, cover plate60 and/or related components, which can be used by the controllers 30,40 and/or 50 to vary the heating/cooling output of the one or moreheating/cooling elements 20/26 in order to keep the temperature of apart of the substrate table WT and/or cover plate 60 (and/or otherrelated components) within a range of one or more corresponding targettemperatures. For example, a feedback loop may be provided that adjuststhe output of the one or more heating/cooling elements 20/26 in order toreduce a difference or differences between the reading(s) of thetemperature sensor(s) and one or more target temperatures.

FIG. 11 schematically shows an embodiment wherein the substrate W issupported on a substrate holder 2 positioned between the substrate W andthe substrate table WT. This arrangement does not comprise any heatingor cooling elements in the substrate holder 2. This means that thesubstrate holder 2 can be made less bulky and complex, which may reducemanufacturing expense. The smaller volume of material may also reduceproblems caused when the substrate holder 2 expands or contracts,because these expansions or contractions will be proportionallysmaller/weaker due to the smaller amount of material involved. Designconstraints relating to thermal properties of the substrate holder 2(e.g. the thermal expansion coefficient) may therefore be relaxedproviding greater freedom to optimize other physical or economicproperties of this component.

Enhanced thermal control of the substrate W, substrate holder 2,substrate table WT and/or cover plate 60 is achieved by providing a highthermal conductance pathway between the substrate W and the substrateholder 2, the substrate holder 2 and the substrate table WT, and/or thecover plate 60 and the substrate table WT. This may be achieved,according to an embodiment, by incorporating a thermally conductingcoupling medium 66 between the substrate W, cover plate 60 and/orsubstrate table WT. In the example shown, this coupling medium is aliquid and is provided in a region beneath the substrate holder 2 andcover plate 60 and is contained by one or more plugs 64. The liquidprovides a large contact surface between the substrate holder 2 and thesubstrate table WT without losing the flexibility associated with theuse of burls as the main supporting mechanism. A liquid with a highthermal conductivity should be particularly effective.

Alternatively or additionally, a gaseous coupling medium may be used.For example, where the region between the substrate table WT and thecover plate 60 and/or substrate holder 2 is held under low pressure(i.e. at a pressure significantly below atmospheric pressure), the lowpressure level may be reduced so as to achieve a balance betweensecuring the substrate holder 2 (or cover plate 60) sufficiently firmlyand providing some gas to improve thermal coupling of the substrateholder 2 (or cover plate 60) to the substrate table WT. Alternatively oradditionally, regions of different gas pressure may be establishedbeneath the substrate holder 2 and/or cover plate 60, a region of lowpressure acting to secure the component(s) while a region of higherpressure acts to improve thermal coupling. Purified air may be used asthe gas coupling medium, for example.

According to an alternative mechanism, a non-fluid coupling medium maybe provided between the burls 3,5 and the substrate W, substrate holder2, cover plate 60 and/or substrate table WT. For example, indium foil,which is very soft and a good conductor of heat, may be used.

The enhanced thermal pathway between the substrate W and/or substrateholder 2 and the substrate table WT and/or cover plate 60 may ensurethat measures taken to stabilize the temperature of the substrate tableWT also act to stabilize the temperature of the substrate W andsubstrate holder 2. This means that thermal expansion/contraction of thesubstrate holder 2, for example, may be less likely to cause distortionof the substrate table WT, which provides more scope for choosing asuitable material for the substrate holder 2. For example, a SiSiCsubstrate holder 2 may be used, which has high wear resistance.

FIG. 12 schematically shows an embodiment comprising a measuring systemto determine a position of a portion of the substrate table WT relativeto a reference frame 92, which may, for example, be rigidly fixed withrespect to the projection system PS and/or lithographic apparatus. Theapparatus also comprises a substrate table distortion determining device86 which is arranged to generate data regarding a distortion of thesubstrate table WT, for example from thermal contraction and/orexpansion due to an unwanted temperature variation. The substrate W isdisplaced (for example scanned) relative to the projection system PS bymeans of a substrate table displacement device 90, which operates underthe control of a substrate position controller 84.

The substrate position controller 84 determines how to move thesubstrate W along a desired trajectory by reference to data input fromthe measuring system and the substrate table distortion determiningdevice 86. The measuring system provides regular updates on the positionof a measured portion of the substrate table WT, from which thesubstrate position controller 84 is configured to derive a position ofthe substrate W. This operation is relatively straightforward if thesubstrate table WT maintains a constant geometry because there will be acorrespondingly constant relationship between the position of theportion of the substrate table WT measured by the measuring system andthe position of the substrate. However, this relationship may change ifthe geometry of the substrate table WT changes, which may lead to anerror in the positioning of the substrate W. According to the presentembodiment, this error may be reduced or avoided by using the output ofthe substrate table distortion determining device 86 to update therelationship between the position of the measured portion of thesubstrate table WT and the substrate position to reflect substrate tabledistortion. This approach may provide an improvement in substratepositioning and, therefore, overlay performance, for example, withoutthe need for substantial additional hardware, such as that associatedwith trying to reduce a temperature irregularity in the substrate tableWT and/or its direct physical consequence.

According to an embodiment, the measuring system comprises a pluralityof planar reflectors 82 mounted on the lateral sides of the substratetable WT. One or more interferometers are provided to measure theposition of the surfaces of the mirrors. The one or more interferometersmay each comprise a radiation source 81 and a radiation detector 83 anda system for comparing the emitted radiation with the received radiationin order to determine a separation between the surface of the mirroronto which the radiation is incident at any one time and a fixed pointrelative to the reference frame 92 associated with the interferometer inquestion. By arranging the mirrors to be oriented in orthogonaldirections, for example, it is possible to determine the position of aportion of the substrate table WT along orthogonal axes.

Distortion of the substrate table WT can cause these mirrors to becomeslightly curved. The substrate table distortion determining device 86provides information about this curvature or “mirror profile” so that itcan be corrected for.

One way in which this may be done is by measuring the curvature of themirrors 82 and/or substrate table WT. This may be done as a calibrationrun to gauge how the substrate table WT deforms during one or aplurality of typical exposure sequence(s). The results of thesecalibration runs may be stored in a memory device 88, which may beaccessed online by the distortion determining device 86 in order toprovide the substrate table position controller with suitablecorrections.

Alternatively or additionally, the measuring system may be provided witha plurality of interferometers (e.g. including pairs of radiationsources and detectors), as shown in FIG. 13, for each of the mirrors 82formed in the substrate table WT. Each interferometer in thisarrangement is capable of measuring the distance from a differentportion of the surface of a mirror relative to the reference frame 92 atany one time and can thereby measure the profile of the substrate tableWT very efficiently. This arrangement could be used quickly to derivecalibration data about the expected distortion of the substrate table WTduring exposure or could be used to provide online data to the substratetable distortion measuring device 86. Additionally or alternatively, thesubstrate table position controller 84 could be configured to take anaverage of the readings of each of the interferometers for each of themirrors in order to obtain a more accurate measurement of the positionof one lateral side of the substrate table WT than would be possibleusing only a single interferometer (in the case where the correspondingmirror is not exactly planar).

Alternatively or additionally, the substrate table distortiondetermining device 86 may be configured to determine an expectedsubstrate table distortion by means of a predictive model. The model maybe based on thermal and mechanical properties of the substrate W,substrate holder 2, substrate table WT and/or any component in thermaland/or mechanical contact with any one or more of these components, aswell an expected power input or output from the lithography radiationand/or evaporation of an immersion liquid from the surface of thesubstrate W and/or cover plate 60, for example. Parameters of the modelmay be tuned by reference to calibration measurements. The expectedpower input or output may be derived from analysis of the energy flowassociated with a particular desired dose pattern or may be derived fromcalibration measurements. Where distortion of the substrate table WT isexpected to arise from a thermally expanded substrate holder 2 pressingagainst the substrate table WT, a simplified model based on uniformexpansion of the substrate holder 2 may be effective.

The surface profile of a substrate table positioning mirror 82 may alsobe determined by a mapping method. For example, in the case where twonominally planar reflectors 82 are provided on different non-parallellateral sides of the substrate table WT, and a single interferometer isprovided for each of the mirrors, the surface profile of a first one ofthe mirrors 82 can be mapped by moving the substrate table WT parallelto a normal to a second one of the mirrors 82 while measuring how aperpendicular distance from the interferometer to the surface of thefirst mirror varies. The process could then be repeated, but moving thesubstrate table WT parallel to a normal to the first mirror 82 in orderto map the profile of the second mirror.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a substrate table arranged tosupport a substrate; a projection system configured to project amodulated radiation beam onto a substrate; a liquid supply systemconfigured to provide a liquid in a region between the projection systemand a substrate during exposure; a cover plate, physically separate fromthe substrate table, positioned radially outside of the substrate duringexposure, and configured to provide a surface facing the projectionsystem that is substantially adjacent to and level with the substrate;and a substrate table temperature stabilization device configured toreduce a temperature deviation of a part of the substrate table from acorresponding target temperature by controlling a temperature of a partof the cover plate.
 2. The apparatus according to claim 1, wherein thetemperature stabilization device comprises a heating element configuredto input heat to a portion of the cover plate, a cooling elementconfigured to extract heat from a portion of the cover plate, or boththe heating element and the cooling element.
 3. The apparatus accordingto claim 1, wherein the temperature stabilization device comprises anetwork of channels embedded within the cover plate and a controllerarranged to control a temperature, a pressure, or both, of a heatexchange fluid within the network of channels in order to reduce atemperature deviation of a part of the substrate table from acorresponding target temperature.
 4. The apparatus according to claim 1,wherein the temperature stabilization device comprises a heating elementand a controller arranged to control the heat output from the heatingelement in order to reduce a temperature deviation of a part of thesubstrate table from a corresponding target temperature.
 5. Theapparatus according to claim 4, wherein the heating element comprises amaterial that undergoes a temperature induced phase transition in theregion of a target temperature, the phase transition causing thematerial to change from a state in which it produces a relatively highheating output below the transition temperature to a state in which itproduces a relatively low heating output above the transitiontemperature.
 6. The apparatus according to claim 5, wherein thesubstrate table temperature stabilization device is arranged to pass anelectric current through the material that undergoes a phase transition,the phase transition being such as to cause the heating element materialto change from a state of relatively low resistivity below thetransition temperature to a state of relatively high resistivity abovethe transition temperature.
 7. The apparatus according to claim 5,wherein the substrate table temperature stabilization device is arrangedto apply a changing magnetic field to the material that undergoes aphase transition, the phase transition being such as to cause theheating element material to change from a magnetically hysteretic statebelow the transition temperature to a non-magnetically-hysteretic stateabove the transition temperature.
 8. The apparatus according to claim 1,further comprising a temperature sensor configured to measure thetemperature of a portion of the substrate table, the cover plate, orboth, and wherein the substrate table temperature stabilization deviceis configured to reduce a temperature deviation of a part of thesubstrate table from a corresponding target temperature using atemperature reading from the temperature sensor.
 9. The apparatusaccording to claim 1, further comprising a substrate holder, positionedbetween the substrate table and the substrate, and arranged to supportthe substrate.
 10. The apparatus according to claim 9, wherein athermally conductive coupling medium is arranged to be provided betweenthe substrate table and the cover plate, the substrate holder, or both.11. The apparatus according to claim 10, wherein the thermallyconductive coupling medium is a fluid, indium, or both.
 12. Theapparatus according to claim 9, wherein the substrate holder is formedfrom a material comprising SiSiC.
 13. A device manufacturing method,comprising: projecting a modulated radiation beam, through a liquid,onto a substrate held on a substrate table; and reducing a temperaturedeviation of a part of the substrate table from a corresponding targettemperature by controlling a temperature of a part of a cover plate, thecover plate physically separate from the substrate table, radiallyoutside of the substrate during projection of the modulated radiationbeam and having a surface that is substantially adjacent and level withthe substrate.
 14. A lithographic apparatus, comprising: a substratetable arranged to support a substrate; a projection system configured toproject a modulated radiation beam onto a substrate; a liquid supplysystem configured to provide a liquid in a region between the projectionsystem and a substrate during exposure; a cover plate, physicallyseparate from the substrate table, positioned radially outside of thesubstrate during exposure, and configured to provide a surface facingthe projection system that is substantially adjacent to and level withthe substrate; and a thermal insulator arranged to reduce heat transferbetween the cover plate and the substrate table in order to providethermal shielding of the substrate table by the cover plate.
 15. Theapparatus according to claim 14, wherein the thermal insulator compriseslow thermal conductance burls on which the cover plate is mounted. 16.The apparatus according to claim 15, wherein the low conductance burlsare arranged to have low thermal conductivity, minimal contact area withthe cover plate, minimal contact area with the substrate table, or anycombination of the foregoing.
 17. A device manufacturing method,comprising: projecting a modulated radiation beam, through a liquid,onto a substrate held on a substrate table; and thermally insulating thea cover plate so as to reduce heat transfer between the cover plate andthe substrate table and thereby enable thermal shielding of thesubstrate table by the cover plate, the cover plate physically separatefrom the substrate table, radially outside of the substrate duringprojection of the modulated radiation beam and having a surface that issubstantially adjacent and level with the substrate.
 18. A lithographicapparatus, comprising: a substrate table arranged to support asubstrate; a projection system configured to project a modulatedradiation beam onto a substrate; a measuring system configured todetermine a position of a portion of the substrate table; a substratetable distortion determining device arranged to provide data regarding adistortion of the substrate table; and a substrate position controllerconfigured to control the position of the substrate relative to theprojection system by reference to the position of a portion of thesubstrate table measured by the measuring system and data regarding adistortion of the substrate table provided by the substrate tabledistortion determining device.
 19. The apparatus according to claim 18,wherein the measuring system comprises: a substantially planar reflectormounted on a lateral side of the substrate table; a radiation sourceconfigured to direct radiation onto a localized area on the surface ofthe reflector; and a radiation detector configured to capture radiationreflected back from the localized area of the reflector and determinetherefrom a distance between the reflector surface and a referencepoint, and wherein the substrate table distortion determining deviceprovides data regarding a surface profile of the reflector caused bydistortion of the substrate table.
 20. The apparatus according to claim19, wherein the substrate table distortion determining device isconfigured to measure a thermally-induced distortion of a part of thesubstrate table, the reflector, or both.
 21. The apparatus according toclaim 20, wherein the substrate table distortion determining devicecomprises a plurality of pairs of radiation sources and radiationdetectors, each configured to determine a distance between a differentportion of the reflector surface and a corresponding reference point andthereby derive the data regarding a surface profile of the reflector.22. The apparatus according to claim 19, wherein the substrate tabledistortion determining device is configured to estimate the surfaceprofile of the reflector based on a predictive theoretical model. 23.The apparatus according to claim 19, wherein the substrate tabledistortion determining device is configured to estimate the surfaceprofile based on calibration data stored in a calibration data memory.24. A device manufacturing method, comprising: projecting a modulatedradiation beam onto a substrate held by a substrate table; determining aposition of a portion of the substrate table; and controlling theposition of the substrate relative to a projection system used toproject the modulated radiation beam by reference to the determinedposition of the portion of the substrate table and data regarding adistortion of the substrate table.
 25. A method of mapping a surfaceprofile of a substrate table reflector in a lithographic apparatus,comprising: providing a first substantially planar reflector mounted ona first lateral side of a substrate table configured to support asubstrate, the first reflector having a normal parallel to a first axis;providing a second substantially planar reflector mounted on a secondlateral side of the substrate table, the second reflector having anormal parallel to a second axis non-parallel with respect to the firstaxis; and moving the substrate table parallel to the first axis whilemeasuring a perpendicular distance from a surface of the secondreflector to a reference point in a reference frame.
 26. The methodaccording to claim 25, further comprising moving the substrate tableparallel to the second axis while measuring a perpendicular distancefrom a surface of the first reflector to a reference point in areference frame.
 27. A device manufacturing method, comprising: mappinga surface profile of a reflector of a substrate table by moving thesubstrate table parallel to a first axis while measuring, in a directionsubstantially parallel to a second axis, a distance from a surface ofthe reflector to a reference point, the second axis being substantiallyorthogonal to the first axis; projecting a modulated radiation beam ontoa substrate; and moving the substrate relative to a projection systemused to project the modulated radiation beam in order to exposedifferent target regions of the substrate, the movement controlled byreference to a position of the substrate, the position being determinedby reference to a measurement of the separation of the substrate tablereflector from a reference point and the surface profile of thesubstrate table reflector.